RFR: 8251549: Update docs on building for Git

[Joe Darcy]

8251549: Update docs on building for Git

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Commit messages:
 - Update building docs for git.

Changes: https://git.openjdk.java.net/jdk/pull/21/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=21&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251549
  Stats: 56 lines in 1 file changed: 0 ins; 39 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/21.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/21/head:pull/21

PR: https://git.openjdk.java.net/jdk/pull/21